The preparation of silicone oils or silicone rubber is carried out by polymerization of cyclic or linear polydimethylsiloxane intermediates which are produced by hydrolysis and polycondensation of dimethyldichlorosilane. The industrial hydrolysis processes are carried out continuously, as is described in H. K. Lichtenwalner and M. M. Sprung, Encyclopedia of Polymer Science, Vol. 12, Wiley & Sons, New York, 1970. The basic reaction proceeds according to the equation EQU (m+n)(CH.sub.3).sub.2 SiCl.sub.2 +[2(m+n)+1]H.sub.2 O.fwdarw.((CH.sub.3).sub.2 SiO).sub.n +HO((CH.sub.3).sub.2 SiO).sub.m H+[2(m+n)]HCl,
forming a complex mixture of cyclic and linear siloxanes. Hydrochloric acid is used as source of the water of reaction required for hydrolysis. The hydrogen chloride formed during hydrolysis is reacted with methanol to give chloromethane and is again used in the synthesis of dimethyldichlorosilane by the direct synthesis process.
In principle, the hydrolysis of dimethyldichlorosilane can be carried out in two ways, which are described in W. Noll, Chemistry and Technology of Silicones, Academic Press, Orlando 1968, chapter 5.1.1. Process variant A is hydrolysis using an excess of water. In this, the water available is regulated in such a way that hydrochloric acid is formed. The crude hydrolysate generally contains 20% to 50% of cyclic and 20% to 80% of linear polydimethylsiloxanes having OH end groups. Process variant B is hydrolysis using a deficiency of water. This gives a crude hydrolysate in which the linear siloxane components have Cl at the ends.
The two process variants A and B differ in the way in which the gaseous hydrogen chloride is obtained. In variant A, azeotropic hydrochloric acid (21% by weight) is circulated, brought to saturation concentration (37% by weight) by hydrolysis of the dimethyldichlorosilane and hydrogen chloride is obtained by distillation. For further processing, the crude hydrolysate has to be freed of amounts of remaining hydrochloric acid. In variant B, the hydrogen chloride is obtained directly in dry and gaseous form. Variant B is thus superior to variant A, but has the disadvantage that a part of the chlorine is lost via the Cl-terminated crude hydrolysate. For further processing, the crude hydrolysate has to be neutralized giving a large amount of acid waste water, i.e., very dilute hydrochloric acid, which has to be discarded. This loss via the acid waste water is unavoidable because scrubbing to neutrality of the Cl-terminated crude hydrolysate cannot be successfully carried out with the limited amount of water which would make possible recirculation of the entire scrubbing acid as water of reaction to the hydrolysis process. The reason for this is that, in the scrubbing process, the polycondensation of the OH-terminated linear siloxane components formed from the Cl-terminated linear siloxane components with a limited amount of water proceeds so rapidly that the resulting mean molecular weight of the neutralized crude hydrolysate is too high for further processing, for example, it exceeds a viscosity of 60 cst. The polycondensation occurs in the scrubbing process if there results an aqueous hydrochloric acid, which accelerates the reaction, in contact with the siloxane phase.